A rocket nozzle is narrow in the middle and wide at both ends. Follow the gas from the high-pressure inlet, through the narrow throat where it reaches the speed of sound, and out of the widening bell as a fast supersonic jet.
A de Laval nozzle is a carefully shaped duct that first narrows and then widens â the classic bell-shaped rocket nozzle. Its two-part shape lets a high-pressure gas expand to a very high speed, so the momentum carried by the exhaust becomes thrust.
From the outside the nozzle is just a bell. What matters is the change in the duct along the flow: where it narrows, where it is narrowest, and where it widens. A cutaway makes those changes obvious.
Gas from the combustion chamber collects in the inlet plenum at high pressure. That pressure is the energy the nozzle is about to convert into speed.
As the duct narrows toward the throat, the gas is squeezed into a smaller area, and it accelerates. By the time it approaches the throat it is moving quickly.
The throat is the narrowest cross-section. Here the gas reaches the speed of sound. That sonic condition is the reason the following widening section can accelerate it further instead of slowing it.
Past the throat the duct widens. Because the flow is already sonic, widening now speeds it up. The gas expands and accelerates through this bell-shaped section to supersonic velocity.
The gas finally leaves as a high-speed jet. The faster the exhaust leaves, the more momentum it carries, and the more thrust the engine produces.
The narrow throat brings the gas to the speed of sound. Past that point a widening passage speeds the flow up rather than slowing it, so the two shapes work together to make a fast exhaust.
It is the minimum cross-section, where the gas reaches the speed of sound. That condition is what allows the diverging section to keep accelerating the gas instead of slowing it down.
The bell is the diverging section. Its curved shape guides the gas as it expands and speeds up, so the exhaust leaves as fast as possible and carries more thrust.
Not always. The best exit size depends on the pressure of the surroundings. A nozzle matched to one altitude can be less efficient at another, which is why designers have to choose.
No. A funnel slows a flow as it widens. Past a sonic throat a nozzle does the opposite and speeds the flow up, which is what makes it useful for rockets.